1. Field of the Invention
The present invention relates to an exhaust gas valve device in an internal combustion engine.
2. Description of the Related Art
A conventional exhaust gas valve device is described, for example, in Japanese Patent Application Laid-open No. 11-166428.
Some of the conventional exhaust gas valve devices include a bearing member made of a carbon material fixed to a valve body in order to prevent the generation of noise to as a result of the turning of the valve shaft and to reduce friction.
When the carbon material is used for the bearing member, however, the concentric accuracy of a pair of bearing members made of the carbon material cannot be enhanced as a result of a difference in the thermal expansion coefficient between a metal material for forming the valve body and the carbon material and/or as a result of the fact that the bearing member made of the carbon material is unsuitable for fixing to the valve body by direct press-fitting. Also, a separate part is required for fixing the bearing member to the valve body, resulting in an increase in the number of parts.
An object of the present invention to provide an exhaust gas valve device in an internal combustion engine which can enhance the concentricity accuracy of the pair of bearing members for supporting opposite ends of the valve shaft, to thereby effectively prevent the generation of noise and to effectively reduce the friction.
To achieve the above object, according to the present invention, there is provided an exhaust gas valve device in an internal combustion engine, comprising: a valve body provided in an exhaust system in the internal combustion engine and defining a flow passage through which an exhaust gas flows, a valve shaft mounted to traverse the flow passage, a valve member mounted to the valve shaft within the valve body, a bottomed cylindrical first bearing member mounted between the valve shaft and the valve body with one end of the valve shaft turnably fitted into the first bearing member, a cylindrical second bearing member mounted between the valve shaft and the valve body with the other end of the valve shaft passed through the second bearing member, and an actuator connected to the other end of the valve shaft protruding from the second bearing member for driving the valve shaft to turn.
The valve body, the valve shaft and the first and second bearing members are formed of metal materials having equivalent thermal expansion coefficients. The first and second bearing members are press-fitted into the valve body. A skin of a graphite-based solid lubricant is formed on a surface of the valve shaft in regions corresponding to the first and second bearing members.
With this arrangement, the first and second bearing members are formed of the metal materials having the thermal expansion coefficient equivalent to the metal material for forming the valve body. Therefore, even if the first and second bearing members are press-fitted directly into the valve body, there is not a possibility that the first and second bearing members are removed from the valve body as a result of a change in temperature. Thus, the first and second bearing members can be fixed to the valve body, while avoiding an increase in the number of parts, whereby the concentricity accuracy of the pair of bearing members can be enhanced. In addition, because the valve shaft is also formed of the metal material having the thermal expansion coefficient equivalent to that of the valve body, clearances between the valve shaft and the bearing members can be minimized. Moreover, because the skin of the graphite-based solid lubricant is formed on the surface of the valve shaft in the regions corresponding to the first and second bearing members, the slidability of the valve shaft at a high temperature can be improved, whereby the generation of noise can be effectively prevented and the friction can be effectively reduced, in cooperation with the enhancement in concentricity accuracy. Thus, it is possible to improve the durability of the exhaust gas valve device.
An expansion graphite ground packing may be interposed between the valve shaft and the second bearing member or a ring-shaped member which is fixed to the second bearing member to surround the valve shaft.
With this arrangement, the leakage of exhaust gas from the periphery of the valve shaft at a high temperature can be prevented by the expansion graphite ground packing having a high heat resistance particularly in an atmosphere basically containing no oxygen, as in an exhaust gas from the internal combustion engine. Moreover, because the expansion graphite ground packing has a low shape restorability, when the deflection of the valve shaft is large, there is a possibility that the sealability of the expansion graphite ground packing is deteriorated. In the present invention, however, the concentricity accuracy of the pair of bearing members can be increased, and the clearances between the valve shaft and the bearing members can be minimized, whereby the deflection of the valve shaft can be suppressed to a smaller level. Therefore, it is possible to maintain the sealability of the expansion graphite ground packing at a high level.